This Week in Science

Science  17 Dec 2010:
Vol. 330, Issue 6011, pp. 1583
  1. Size Matters


      Animals regulate their growth so that all organs are mutually proportional, even when growth occurs at different times. However, the mechanism by which this occurs is unknown. Nijhout and Grunert (p. 1693, published online 25 November) performed an analysis of relative wing growth in the tobacco hornworm, Manduca. Animals that were small owing to starvation had smaller wings, due to their slower growth rate, and they also stopped growing earlier than large, well-fed larvae. The insect hormone ecdysone was implicated in the process that governs this scaling relationship between adult wings and body size.

    1. Unlikely Planet

        Most known extrasolar planets orbit stars similar to the Sun. Very few planets have been detected around metal-poor stars whose abundances of elements other than hydrogen and helium are much lower than those of the Sun, or around stars that are at a late stage in their evolution. Setiawan et al. (p. 1642, published online 18 November) report the detection of a close-in giant planet around a metal-poor star belonging to a group of stars that formed in a satellite galaxy of the Milky Way. The star has gone past the red giant phase of stellar evolution, when stars like the Sun expand up to many times their original size, and so it is unclear why the planet was not engulfed by the star as it expanded.

      1. Spin Control

          Controlling and manipulating the spin of an electron is a central requirement for applications in spintronics. Some of the challenges researchers are facing include efficient creation of spin currents, minimization of Joule heating, and extending the lifetime of electronic spins, which is especially important for quantum information applications. Costache and Valenzuela (p. 1645) address the first challenge by designing and fabricating an efficient and simple superconducting-based single-electron transistor that can produce spin current with controlled flow. Key to the design is asymmetric tunneling, which leads to a ratchet effect (or diode-like behavior), allowing the separation of up and down spins. Jonietz et al. (p. 1648) use electric currents five orders of magnitude smaller than those used previously in nanostructures to manipulate magnetization in a bulk material, MnSi, pointing the way toward decreased Joule heating in spintronic devices. This so-called spin-torque effect causes the rotation of the skyrmion lattice of spins, characteristic of MnSi, which is detected by neutron scattering. Finally, McCamey et al. (p. 1652) extend the short lifetime of an electronic spin of a phosphorous dopant by mapping it onto the much longer lived nuclear spin of the atom. Mapping the nuclear spin back onto the electronic spin allows production of a spin memory with a storage time exceeding 100s, which should prove useful for future practical applications.

        1. Better Imaging When Separated

            CREDIT: GHOSH ET AL.

            A fluorescent probe works better if its absorption and emission wavelengths are well separated; otherwise, the probe tends to reabsorb its own emission. Ghosh et al. (p. 1656, published online 25 November) found that oxygen doping of semiconducting single-wall carbon nanotubes (SWCNTs) improved the characteristics of these materials as imaging probes in the near-infrared. Exposure of SWCNTs to ozone and then to visible light caused the emission wavelength to be 10 to 15% longer than the absorption wavelength. They imaged these probes and untreated SWCNTs in cultured human cells and found an ∼20-fold improvement in contrast.

          1. Metabolism Without Modification

              Obesity-associated metabolic disease has rapidly become a public health priority in the developed world and is being addressed through prevention strategies aimed at lifestyle changes and through pharmacological approaches. Barnett et al. (p. 1689, published online 18 November) designed a drug that inhibits the action of ghrelin, a circulating peptide hormone that increases fat mass and food intake. The drug, a bisubstrate analog called GO-CoA-Tat, is a selective antagonist of ghrelin O-acyltransferase (GOAT), an enzyme that catalyzes a posttranslational modification that is essential for ghrelin activity. Injection of GO-CoA-Tat into wild-type mice on a high-fat diet improved glucose tolerance and reduced weight gain, probably through changes in metabolic activity. Because GO-CoA-Tat is a peptide-based drug that requires repeated injection, it is unsuitable for clinical use, but GOAT does represent a potentially valuable target for future drug development efforts in metabolic disease.

            1. Burn, Baby, Burn

                The atmospheric trace gas, carbon monoxide, has important effects on methane and on ozone, and is important both in atmospheric chemistry and for its indirect influence on climate. Little is known about the abundance and sources of CO prior to the industrial age, or about the importance of anthropogenic activities on its budget. Wang et al. (p. 1663, published online 2 December; see the Perspective by Prentice) present a 650-year-long record of CO atmospheric concentration and isotopic composition, using samples from Antarctic ice cores, in order to begin to reconstruct past CO variability and its causes. The concentration of CO decreased by ∼25% from the mid-1300s to the 1600s, and then recovered completely by the late 1800s. Large variations in the degree of biomass burning in the Southern Hemisphere are likely to have been primarily responsible for the observed changes.

              1. A Hot Dipole

                  In a ferroelectric material, there is an alignment of local electric dipole moments that produces a net overall electric polarization. This state is accompanied by a decrease in symmetry, which can be restored by heating above a critical temperature. In contrast, through a combination of theory and experiments, Božin et al. (p. 1660) now show that with increasing temperature, rock-salt–structured lead telluride and lead sulfide go through a phase transition from a high symmetry phase to a low symmetry phase with an associated dipole moment. Paradoxically, the dipoles are stabilized in the disordered state at high temperature, even though the undistorted structure has lower internal energy.

                1. Dissecting Nitric Oxide Reductase

                    CREDIT: HINO ET AL.

                    Bacterial breakdown of nitrogen compounds in soil and the oceans provides the largest emission source of the greenhouse gas, nitrous oxide (N2O). A key enzyme in this process is nitric oxide reductase (NOR), which catalyzes the reduction of nitric oxide (NO) to N2O. Hino et al. (p. 1666, published online 25 November; see the Perspective by Moënne-Loccoz and Fee) now describe the crystal structure of NOR from Pseudomonas aeruginosa. Consistent with their evolutionary relatedness, the transmembrane region topology and arrangement of metal centers in NOR are similar to those in cytochrome oxidases, key enzymes in aerobic respiration.

                  1. Few But Powerful

                      Drug activation of the different types of acetylcholine receptors in cholinergic neurons often generates opposing or conflicting effects. Using optogenetic techniques in transgenic mice, Witten et al. (p. 1677) investigated the function of a rather enigmatic subpopulation of cholinergic neurons, the giant interneurons of the nucleus accumbens. Their excitation paradoxically reduced neighboring medium spiny neuron firing, while their inhibition increased medium spiny neuron firing. Furthermore, the giant interneurons were directly activated by cocaine, and silencing their drug-induced activity during cocaine exposure in freely behaving animals disrupted cocaine reward.

                    1. Making a Germ Cell

                        When it comes to generating the germ line, animals fall into two classes: those, like mammals, which use inductive interactions to specify the germ line and those, like nematodes, which use germ plasm—a specialized egg cytoplasm that segregates asymmetrically in embryos. Germ plasm contains germ, or P, granules—RNA-protein aggregates that have been thought to harbor the germ line determinants. Now, Gallo et al. (p. 1685, published online 2 December) describe a Caenorhabditis elegans mutant that challenges this belief. The germ line still formed even when germ granule components were missegregated. Thus, even in animals with germ plasm, germ granules appear to be a consequence, not a cause, of germ cell specification.

                      1. Lymphocytes Layer It On

                          Cells of the immune system begin to develop from hematopoietic stem cells (HSCs) during fetal life. In the adult, HSCs continue to produce immune cells to replenish dying cells or in response to an infection. In mice and birds, immune cell development occurs in a “layered” manner, whereby distinct populations of HSCs that arise at different times during development generate distinct immune cell lineages. In contrast, development of human immune cells, and T lymphocytes in particular, is thought to be linear. Mold et al. (p. 1695; see the Perspective by Betz) now show that T lymphocyte development in humans is also “layered,” and strategically so. T cells that arise from fetal HSCs are enriched in regulatory T cells, which promote immune tolerance, rather than classical T cells, which readily respond to foreign antigen. By favoring the development of regulatory T cell populations during fetal life, the immune system is perhaps better able to keep responses to maternal antigens in check. The development of large numbers of classical T cells is delayed until after birth when infectious agents represent a more imminent threat.

                        1. Beyond the Greatwall

                            Protein phosphorylation and dephosphorylation provide a central mechanism that controls the eukaryotic cell division cycle and entry of cells into mitosis. A form of protein phosphatase 2A (PP2A) has an important role inhibiting phosphorylation-dependent activation of cyclin-dependent kinase 1 (CDK1) itself and also dephosphorylating substrates of the active CDK1 that promote mitosis. PP2A activity is inhibited when another protein kinase, known as Greatwall, is activated (see the Perspective by Virshup and Kaldis). Mochida et al. (p. 1670) and Gharbi-Ayachi et al. (p. 1673) searched for substrates of Greatwall that might participate in the cell cycle regulatory machinery. When phosphorylated by Greatwall, a pair of small related proteins, Arpp19 and α-endosulfine, inhibited activity of PP2A. These effects were critical for regulation of mitosis in Xenopus egg extracts and in human cancer cells. Greatwall itself is phosphorylated and activated by CDK1—thus, apparently contributing to a feed-forward loop that contributes to the switchlike commitment of cells to mitosis.

                          1. Essential and New

                              Genes can be broadly grouped into two sets on the basis of their contribution to fitness: those that are essential to the life of an organism and those that can be dispensed with. However, the degree of essentiality in evolutionarily “new” genes—genes that have originated in the recent past—is unknown. Chen et al. (p. 1682) investigated the origination and evolution of new genes within 12 Drosophila species and found, surprisingly, that over one-third of genes that have originated within the last 3.5 million years show essential function and that these functions are overrepresented during larval development. Approximately the same proportion of older genes was essential, although many of these genes also appear to show enrichment at later developmental stages. These findings challenge conventional wisdom that would claim that essential genes are ancient and conserved among animal taxa.